multiplier屬性是只讀的，不可以進(jìn)行直接修改。stackoverflow給出答案，特意分享出來，給有需要的人。原網(wǎng)站在這里

OC寫法

- (void)p_changeMultiplierOfConstraint:(NSLayoutConstraint *)constraint multiplier:(CGFloat)multiplier {
    
    [NSLayoutConstraint deactivateConstraints:@[constraint]];
    NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:constraint.firstItem attribute:constraint.firstAttribute relatedBy:constraint.relation toItem:constraint.secondItem attribute:constraint.secondAttribute multiplier:multiplier constant:constraint.constant];
    newConstraint.priority = constraint.priority;
    newConstraint.shouldBeArchived = constraint.shouldBeArchived;
    newConstraint.identifier = constraint.identifier;
    [NSLayoutConstraint activateConstraints:@[newConstraint]];
}

調(diào)用

        [self p_changeMultiplierOfConstraint:_proImgWidthCtt multiplier:0.5];

Swift寫法

import Foundation
import UIKit
extension NSLayoutConstraint {
    /**
     Change multiplier constraint

     - parameter multiplier: CGFloat
     - returns: NSLayoutConstraint
    */
    func setMultiplier(multiplier:CGFloat) -> NSLayoutConstraint {

        NSLayoutConstraint.deactivate([self])

        let newConstraint = NSLayoutConstraint(
            item: firstItem,
            attribute: firstAttribute,
            relatedBy: relation,
            toItem: secondItem,
            attribute: secondAttribute,
            multiplier: multiplier,
            constant: constant)

        newConstraint.priority = priority
        newConstraint.shouldBeArchived = self.shouldBeArchived
        newConstraint.identifier = self.identifier

        NSLayoutConstraint.activate([newConstraint])
        return newConstraint
    }
}

原文鏈接